The present invention relates to electronically agile multi-beam radars in general, and more particularly to clutter positioning apparatus for disposition therein to adaptively position an identified group of clutter signals about a prespecified doppler frequency in the doppler frequency spectrum for each of the beams of the radar.
A typical radar of the pulse doppler-coherent type is depicted in the block diagram schematic of FIG. 1. In the exemplary radar of FIG. 1, the transmission portion includes the conventional units of a mixer/filter circuit 10, an amplifier A1, another mixer/filter circuit 12 and a transmitter unit 14, all cascadedly coupled together to effect a pulsed R.F. signal over signal line 16 coupled to one input of a conventional microwave circulator 18. An antenna system shown at 20 may be coupled to another port of the circulator 18. Still another port of the circulator 18 couples the antenna system 20 to the receiving portion of the exemplary radar which includes an R.F. amplifier, a mixer/filter circuit 24, a first IF amplifier IF1, another mixer/filter circuit 26 and a second IF amplifier IF2, all cascadedly coupled together. A stable local oscillator (STALO) 28 may provide a fixed IF signal denoted as LO2 to both of the mixer/filter circuits 10 and 26. An R.F. signal may be generated by the STALO 28 and provided to a frequency synthesizer 30 over signal line 32. The synthesizer 30 may alter the generated R.F. signal so as to provide a desired sequence of R.F. frequency signals, denoted as LO1, to the mixer/filter units 12 and 24.
Downstream of the IF2 amplifier may be a pair of a conventional mixer/filter circuits 36 and 38 for effecting the in phase (I) and quadrature (Q) signal components from the receiver signals. The STALO 28 may generate a fixed frequency signal denoted as LO3 which is supplied in phase to the mixer/filter circuit 36 and 90 degrees out-of-phase to the mixer/filter circuit 38. The I and Q components of the receiver signals may be supplied to a conventional doppler signal processing section 40 which derives a substantially representative doppler frequency spectrum of signals therefrom.
Generally within the doppler frequency spectrum of signals there may be an identified group of clutter signals, commonly referred to as the main beam clutter, which may be positioned about an undesirable doppler frequency or within an undesirable range of doppler frequencies. Under these conditions, any moving targets having a doppler frequency characteristic within the clutter range would be masked by the clutter signals. In addition, as a result of practical unbalance of hardware components, such as an unbalance between the mixer/filter circuits 36 and 38, for example, which effect the I and Q signal components of the received signals, an image of the main beam clutter may be produced in the doppler spectrum mirrored about the baseband doppler frequency which in the present case is zero doppler frequency. An illustration of these clutter signal bands both main beam and image, is shown in the exemplary graph of FIG. 2A. As has been explained above, any moving targets having doppler frequencies within the ranges of the clutter signals could be masked and pass undetected by the radar.
Present radars are using clutter positioning techniques to tune the main beam clutter to the doppler baseband frequency and remove it from the moving target detection ranges in the doppler frequency spectrum. For example, in an airborne radar employing downlook air-to-air modes, the doppler frequencies of the main beam clutter are a function of primarily the velocity of the aircraft and the angle position of the antenna radar beam with respect to the earth. Thus, with these two factors being either known or measurable, a compensating frequency signal may be injected into the radar at a prespecified location to tune the main beam clutter signals to a new, more desirable, position in the doppler frequency spectrum, normally about the doppler baseband frequency.
One way of accomplishing this is using a conventional clutter position computer 42, a digital-to-analog (D/A) converter 44 and a voltage controlled crystal oscillator (VCXO) 46 as depicted in the schematic of FIG. 1. For example, a signal 48 representative of the measured aircraft velocity and a signal 50 representative of the angle of the radar beam may be provided to the clutter position computer 42 which may then compute therefrom a control word 52 which may be converted to an analog signal 54 via the D/A converter 44 to govern the output frequency of the VCXO 46. The output frequency signal 56 in the present example is provided to the mixer/filter circuit 10.
For an operational example, let us assume that the VCXO 46 generates a frequency signal 56 with a nominal frequency of 40 megahertz. If the fixed frequency signal LO2 is on the order of 1460 megahertz, then the mixer 10 may effect an IF frequency of on the order of 1500 megahertz which is, in turn, amplified by A1 and provided to mixer 12. Should the frequency LO1 be generated at 7500 megahertz, then the mixer 12 provides an R.F. carrier on the order of 9 gegahertz which is pulsed by the transmitter unit 14 and transmitted to the antenna system 20 via line 16 and circulator 18. Accordingly, an echo signal may be received by the antenna system 20, passed through circulator 18 and provided to the R.F. amplifier. The received signal may be beat down in mixer 24 by the signal LO1, which is set at 7500 megahertz. The first IF signal may be conditioned by the IF1 amplifier and further beat down in mixer 26 to the second IF level by the fixed frequency signal LO2. The second IF signal which is now on the order of 40 megahertz may be conditioned by the amplifier IF2 and conducted to the I-Q mixers 36 and 38. If the STALO 28 generated frequency signal LO3 is set at 40 megahertz also, then any doppler frequency spectrum derived from the received echo signals or components thereof will include a main beam and image groupings of clutter signals as illustratively shown in FIG. 2A.
However, the present system includes the clutter position computer 42 which is provided with the signals 48 and 50 to derive a compensating frequency to reposition the main beam clutter to the baseband level within the doppler frequency spectrum. It does this by deviating the frequency signal 56 of the VCXO 46 from its nominally chosen value, in this case 40 megahertz, as governed by the control word 52 via D/A 44. Of course, as the main beam clutter grouping of signals is positioned to baseband, its mirror image in the doppler frequency spectrum is likewise tuned to the same new position. The graph of FIG. 2B illustratively shows the main beam and image clutter groupings of doppler signals positioned about the doppler baseband frequency (i.e. zero doppler frequency).
More sophistication in clutter positioning may be provided in some radars as the application demands. Techniques such as clutter tracking, for example, are presently employed in some radar systems to stabilize the main beam clutter about baseband. In these more sophisticated radars, a "servoing" or additional governing signal 60 may be supplied to the clutter position computer 42 to alter the control word 52 and eventual frequency signal 56 in a timely manner to stabilize the clutter frequency signal grouping about the doppler baseband frequency in the doppler frequency spectrum.
These types of clutter positioning techniques have been found adequate for conventional radars using slow moving mechanically scanned antennas wherein the signal beam is scanned through a spatial region very slowly. However, the more contemporary radars are designed for electronically agile multibeam operation where the antenna beam is electronically and rapidly switched between a multiplicity of targets, that is the antenna power is time shared through a variety of antenna beam directions. Thus, the main beam clutter doppler frequency groupings will be at various doppler frequencies dependent primarily on the direction of the antenna beam and aircraft velocity.
In the clutter positioning operation, each main beam clutter grouping in the doppler spectrums of the plurality of antenna beams will have to be tuned to the baseband doppler independently. If the clutter positioning is mechanized using the VCXO embodiment for clutter positioning as described in connection with the embodiment of FIG. 1, a clutter positioning circuit comprising the elements 42, 44 and 46 would be needed for each directionally effected antenna beam of the radar. Rapidly tuning a VCXO is not suitable since phase memory must be maintained in a coherent radar.
To better understand the problem of phase memory, let us assume that each beam of the antenna includes at least one transmission of a plurality of R.F. pulses constituting a radar look and that the radar is operative to receive echo R.F. pulses of the look for each transmitted beam interspersed in time with echo R.F. pulses of the looks of the other transmitted beams of the plurality of beams. The interpulse period of the pulse doppler coherent radar may be divided into transmissionable time slots with each time slot corresponding to a potential transmission time for an R.F. pulse of a different beam look. Under these conditions, the radar must keep a fixed phase relationship from pulse-to-pulse in order to derive a substantially representative doppler frequency spectrum of signals associated with each look of the radar beams. Thus, with the requirement of a phase memory another problem is introduced further complicating the clutter positioning operation of a electronically agile multibeam radar.